a. Field to which the invention pertains
In the manufacture of paper, a dilute aqueous slurry is deposited on a moving foraminous belt. When the water drains away, the cellulose fibers in the slurry form the resulting paper sheet.
In order to obtain a sheet which has the desired specifications, e.g. thickness and the basis weight, it is imperative that the quantity and velocity of the slurry exiting from the headbox be controlled. In order to control the quantity and velocity of the slurry exiting from the headbox, the total head must be controlled since this determines the flow from the headbox for a given headbox slice opening. Since the level of stock within the headbox determines part of or all of the total head, it in turn becomes necessary to control the amount of stock which is delivered to the headbox, since the amount of stock flowing to the headbox controls the level of stock within the headbox. Control of this important variable, i.e. stock flow to a paper machine headbox, is the field to which this invention pertains.
B. Prior Art
Control of stock flow to a paper machine headbox is essentially a flow control problem. Thus, the prior art has resorted to the usual expedients employed in solving flow control problems. For example, the stock flow rate to the headbox may be measured by appropriate means, e.g. a magnetic flow meter, and compared to a set point representing the desired flow rate. If the actual stock flow rate deviates from the desired stock flow rate, appropriate control action is taken which, for example, might comprise either varying the speed of the stock pump or, alternatively, maintaining the stock pump at a constant speed and manipulating a flow control valve downstream of the stock pump. Although the prior has utilized this approach, certain disadvantages have been recognized. For example, in a large paper machine extremely high stock flow rates are encountered, e.g. flow rates as high as 25,000 gpm are not uncommon. Thus, if a flow control valve is placed in the main stock line, a small change in the valve position will result in a gross change in the stock flow rate. The resulting high gain between the stock flow control valve position and the total head creates at least two problems. First, commercially available valves are not infinitely adjustable, i.e. there is a minimum finite amount by which the position of such a valve can be changed. Although this amount is small and acceptable in most environments, the high flow rate and the high gain associated with the valve used as above precludes fine control of the stock flow because the change in flow rate associated with the minimum finite change which the valve can undergo is quite high. Second, because of the high gain between the stock flow valve position and the total head, the gain of the control loop positioning the valve must be very low (very high proportional band) in order to avoid instability. As is well known to those skilled in the art, a low control loop gain or high proportional band setting reduces the sensitivity of the control loop and obviates fine control. Thus, with this approach, it is difficult if not impossible to achieve fine control of the stock flow.
In an effort to circumvent the sensitivity problem created by locating a flow control valve directly in the stock line, the prior art utilized the approach of shunting the stock pump with a by-pass line and, rather than directly controlling the stock flow, controlled the stock flow by controlling the by-pass flow with a by-pass flow control valve. With this approach, a change in the position of the by-pass flow control valve resulted in a smaller change in the stock flow than would have occured with a valve inline with the stock pump. Thus, it was possible, under most circumstances, to utilize a lower proportional band or higher gain in the stock flow controller. However, even with this approach, when the by-pass flow control valve approached one of its operating limits, i.e. full open or full closed, any further small changes in the position of the by-pass flow control valve resulted in large changes of the stock flow thus once again creating a sensitivity problem. Further, if the by-pass flow control valve actually reached one of its operating limits, i.e. was actually either full open or full closed, further control of the stock flow was impossible without some manual manipulation of the speed of the stock pump.
In summary, all of the prior art approaches to solving the problem of controlling the stock flow to a paper machine headbox have been attended by certain disadvantages which are usually characterized by the necessity of reducing loop sensitivity (higher proportional band) in order to maintain acceptable stability. Through the use of my invention as hereinafter described, the operational disadvantages of prior art control systems have been eliminated.